Mass Transfer Characteristics of Solvent
Extraction into a Single Drop at the Tip of a Syringe Needle

The amount of a sample compound extracted into a 1-L drop of n-octane
suspended in a stirred aqueous solution from the tip of a microsyringe
needle is measured by gas chromatography (GC) as a function of time. The
observed extraction rate curve is first order and yields the overall mass
transfer coefficient for the sample compound, o. For a given compound,
o varies linearly with stirring rate. Among the four compounds malathion,
4-methylacetophenone, 4-nitrotoluene, and progesterone, at a given stirring
rate, o is linearly proportional to the diffusion coefficient of the compound
(Daq). This supports the film theory of convective-diffusive mass transfer,
as opposed to the penetration theory. The relative standard deviation
of the GC signal for 4-methylacetophenone after a 1.00 min extraction
at 1500 rpm is 1.5%, which suggests that the system exhibits excellent
potential as a tool for rapid analysis by solvent extraction/GC.

Abstract Extractions of liquid samples were carried out using wall coated
needles prepared from stainless steel capillary columns instead of syringe
needles. This micro extraction technique was applied to the analysis of
pesticides in water. Important parameters influencing the extraction such
as sample velocity, extraction time and also the desorption parameters
were investigated and optimized. Automation of this technique was realized
using a conventional automatic sampler. Limits of detection were improved
using the multiple extraction / desorption technique. Chromatographic
data and limits of detection were compared with those obtained by solid
phase micro extraction (SPME). Using a needle with a 7 wm film yielded
limits of detection varying from 0.001-0.1 wg/L and were in the same range
as those resulting from the extraction using a 100 wm polydimethylsiloxane
(PDMS) SPME fiber. The main advantages of the needle extraction technique
were the significantly higher extraction speed and the practical aspects
of a stable steel needle compared to those of a fragile fiber. The extraction
speed using a needle with a ¶7 wm film was up to five times higher than
the speed of SPME using a 100 wm PDMS fiber. The steel needle could be
stressed mechanically in a higher extent than a SPME fiber. Sample volumes
and aliquots of liquid media could be handled and moved from one bottle
to another using the automatic sampler.